Twisting is an important step of short fibre spinning. In this process, the yarns, are elastically twisted and transformed to attain sufficient strength, wear resistance and smoothness. However, as a negative effect, a large amount of residual torque or twist liveliness is also brought about in the yarns simultaneously. Such twist liveliness of the yarns renders a significant influence on the possessing quality of the latter products. For example, if yarns with twist liveliness are used on knitting, loops of the fabric will lose their balance because of the variation of torsion stress in the yarns. In order to attain the natural structure with the minimum energy condition, the loops tend to rotate to release the internal torsion stress. As a result, one end of the loops will tilt and protrude from the fabric surface, while the other end will stay inside the fabric. Such deformation of the loops will increase the spirality of the fabric; a deformation similar to the rib effect, which should be prevented to the utmost in the spinning industry. Thus, the balancing of torque inside the yarns is particularly important.
Yarns are made from a large quantity of fibres polymerized by their friction inbetween. Hence, the residual torque of the yarns or the spirality of the fabric is mainly affected by said characteristic of the fibres, such as the type and cross sectional shape of the fibres, the polymerizing manner of the fibres and the internal structure of the yarns, etc.
First of all, different types of fibres have a different modulus (i.e. tensile, bending and shear) and cross sectional shape, thus lead to different degree of stress in the yarns. According to the report of Arauj and Smith in the Textile Research Journal, Vol. 59, No. 6, 1989, in the cotton/polyester blended yarns, increasing the ratio of polyester will enhance the twist liveliness of rotor and ring yarns, thus improving the spirality of the yarns. This is because polyester has a higher modulus, and said two types of fibre has different cross sectional shapes.
Next, different yarn structures have a different distribution of stress. Experimental results, such as Barella and Manich in the Textile Research Journal, Vol. 59, No. 12, 1989, Lord and Mohamed in the Textile Research Journal, Vol. 44, No. 7, 1974 and Sengupta, and Sreenivasa in the Textile Research Journal, Vol. 64, No 10, 1994 show that, friction yarns (DREF-II) has the largest residual torque and trend of deformation in the priority sequence as ring yarns, rotor yarns and air-jet yarns. The different residual torques of said four types of yarn show the difference among their structures. It is generally agreed that single ring yarns are composed of a plurality of uniformly enveloped concentric helical threads, which fibre migration is a secondary feature. Hence, when the ring yarns are reverse-twisted, their strength will gradually decrease to zero, by then the yarns will be all dispersed. In relative to ring yarns, unconventional spinning system produce yarns with core-sheath structures, such as rotor spinning yarn, air jet spinning yarn and friction spinning yarns. The packing density of said yarns is uneven, mainly characterized in the partial entanglement and enwrapment of the fibres. As a result, during reverse twisting, the strength of said yarns would not be completely disappeared, as disclosed in the Textile Research Journal, Vol. 58, No. 7, 1988 by Castro etc.
In addition, many factors can affect the degree of movement freedom of the loops of the fabric and also the final spirality of the fabric. Said factors include fabric structure, parameters of the knitting machine, and the fabric relaxation and fabric setting due to finishing. All the aforesaid factors affecting the spirality of fabric are reported in detail as disclosed by Lau and Tao in the Textile Asia, Vol. XXVI, No. 8, 1995.
Same as other materials, the residual torque of the yarns can be reduced or eliminated with different methods. In the past several ten years, a variety of torque balancing methods have been developed. According to basic theory, they can generally refer to two categories: permanently processing method and physical torque balancing method.
Permanently processing method mainly accomplishes the purpose of releasing residual torque by transforming the elastic torsional deformation into plastic deformation. Said method mainly relates to all sorts of processing technique of material, such as thermal processing, chemical processing and wet processing etc. In the Textile Research Journal, Vol. 59, No. 6, 1989, Araujo and Smith have proved that in relative to air-jet and rotor yarns, the heat processing of single cotton/polyester blended yarns can effectively reduce the residual torque of the yarns, However, in relative to natural fibres such as cotton or wool, permanent processing is too complicated. It may involve stream processing, hot water processing and chemical processing (such as mercerization in the case of cotton yarns and treatment with sodium bisulphite in case of the wool yarns) In addition, in relative to natural yarns, permanent processing cannot completely eliminate the residual torque of the single yarns; meanwhile it may cause damage and abruption to the yarns.
In relative to permanent processing, physical torque balancing is a pure mechanical processing technique. The main point of said method is fully utilizing the structure of yarns to balance the residual torque generated in different yarns while maintaining the elastic deformation characteristic of the yarns. Currently in the industry, separate machines are required to enforce torque balancing of the yarns; the cost is thus higher. Said method comprises plying two identical singles yarns with a twist equal in number but in the opposite direction to that in the singles yarns; or feeding two singles yarns with twist of the same magnitude but in opposite direction onto the same feeder.
Recently, some new torque balancing methods for yarns also emerges. In the Textile Research Journal, Vol. 65, No. 9, 1995, Sawhney and Kimmel has designed a series spinning system for processing torque-free yarns. The inner core of said yarns is formed by processing with an air-jet system while outside the core is enwrapped with crust fibres similar to DREF-III yarns. In the Textile Research Journal, Vol. 62, No. 1, 1992, Sawhey etc. have suggested a method of processing ring cotton crust/polyester inner core yarns Said yarns accomplish balancing condition by utilizing core yarns with opposite twisting direction from synthetic yarns, or applying heat processing on the polyester portion of said yarns. However, it is readily seen that the machines and processing techniques related to the aforesaid method are generally more complicated. In the Textile Research Journal, Vol, 57, No. 10, 1997, Tao has processed the layer structure of the inner core-crust of rotor yarns to generate torque-free single yarns, yet said technique is not suitable for ring yarns.